There are certain applications where continuity of acquisition during a measurement process has importance for either temporal (as in transient data collection) or phase (as in phase variation or clock recovery) coherence. In the temporal case, there are examples of pulse measurements of a device under test (DUT) where it can be desirable to acquire pulse response at a series of frequencies or powers (e.g., a countable number per pulse or a countable number of pulses per step), but no pulses can be skipped in order to collect the transient response of the DUT. Such examples include high power radar systems for air traffic control or electronic surveillance or other application where a system must be very quickly turned on or turned to a specific target. There is a need to understand how a transmitter is acting in the first few time increments after it is turned it. In such higher power scenarios thermalisation and other transients are more likely to occur and cause problems.
Currently some systems use pure time domain acquisition and sort through the data record for the relevant parts, but the base dynamic range can be limited, corrections for match and other defects difficult, and identification of the points of interest complicated. Further, the allowed sweep speed may be quite limited because of triggering complexities and if both sweep and acquisition are not carefully controlled the DUT can be exposed to energy that is not characterized or information can be lost.
In the phase coherence category, one application is that of embedded local oscillator (LO) measurements. Measuring the phase response or group delay of a frequency converter when the LO of a DUT is not precisely known has been a difficult problem for decades. Some solutions involve a modulated stimulus, such as narrow-band frequency modulation (NBFM) or double-sideband suppressed-carrier transmission (DSBSC) amplitude modulation (AM), where the phase relationship between the sidebands of modulation is used to deduce the phase of the output signal. These methods often suffer from poor phase resolution. Other techniques involve a phase-locking/phase-hunting method on the measurement receiver to follow the DUT output frequency. These methods often can only handle a fairly small DUT frequency error and are prone to losing the phase reference as the drift rate changes even by a small amount.